As disclosed in the specifications of U.S. Pat. Nos. 3,317,322 and 3,761,276, it is known that reverse images are obtained by development in the presence of fogging agents or by uniform exposure during development of photographic light-sensitive materials containing internal latent image type silver halide grains, silver halide grains whose grain surfaces have been chemically sensitized, comprised of an internal silver halide nucleus (core) which is either doped with metal ions or chemically sensitized, or which has undergone both treatments, and an external silver halide shell covers at least the light-sensitive sites on the said internal nucleus (hereinafter referred to as core/shell grains).
However, the chemically sensitized nuclei produced by the chemical sensitization of the surfaces of such internal latent image type core/shell silver halide grains have poor age stability and have the disadvantage that the maximum density (Dmax) of the reverse image varies if after the internal latent image type core/shell silver halide grains having these chemically sensitized nuclei have been stored for a long time (for example, 10 days or more) at low temperatures (hereinafter referred to as cold-storage aging) said stored silver halide grains are introduced into direct positive photographic light-sensitive materials.
In order to obtain a sufficiently high Dmax in comparison to the minimum density (Dmin) of the reverse image, the chemical sensitization of the surfaces of internal latent image type core/shell silver halide grains is carried out using various methods. Also, it is known that the extent of chemical sensitization has an effect on negative image speed and on Dmin. Furthermore, the extent of surface chemical sensitization also has an effect on variations in Dmax and Dmin caused by cold-storage aging of the emulsions. It is thought that these variations caused by cold-storage aging are partially due to changes in the surface chemically sensitized nucleus during low temperature storage. Accordingly, in order to improve the cold-storage aging properties, it is useful to ensure that surface chemical sensitization is completed during that stage, and that changes in performance do not occur after that (namely, during emulsion storage, during the preparation of the coating solution or during the aging of the light-sensitive materials) due to changes in the sensitized nucleus or the effects of unreacted sensitizing agents remaining. By way of example, the method in which surface chemical sensitization is carried out in the presence of a polymer such as poly(N-vinylpyrrolidone) as disclosed in JP-B-60-55821 (the term "JP-B" as used herein means an "examined Japanese patent publication"), the method in which surface chemical sensitization is carried out after deactivating the additives used in the silver halide grain formation using deactivating agents as mentioned in JP-A-61-3137 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and techniques such as changing the pAg of the surface chemical sensitization disclosed in Japanese Patent Application No. 63-40479 have been considered as methods of surface chemical sensitization which are in accordance with these requirements. However, none of these give fully satisfactory results.